Canon 50mm f1.4 v 85mm f1.2

I have been thinking about a prime for some shallow depth of field use, including portraiture.

I am aware that 50mm is not a classic length for portraits (this is on a full frame 5DIII by the way) but working indoors in confined spaces at times 50mm can be useful.

Anyway, despite lusting after the 85mm f1.2, prices have recently crept up and so before forking out nearly £1,400 on that, I bought the 50mm f1.4 as it is pretty cheap (£220) and worth trying.

Now my dilemma. At f1.4 this lens will achieve very shallow depth of field (e.g. eyes in focus, ears pretty soft) but it is also quite soft overall. I have not carried out micro adjustment. Also autofocus is a bit erratic even in good light. Sharpness improves a lot by f2.8, but if I actually want very shallow depth of field, sharpness would be nice as well!

The attraction of this lens is that it is over £1,100 cheaper than the 85mm f1.2 and a great deal smaller and lighter. However, I don't actually want to compromise image quality and am quite happy to send the 50mm back IF the 85mm will be a big leap forward.

Attached are two examples of a "snap" i.e. not trying to compose anything, just trying the lens across a restaurant table indoors and out. This is my long suffering wife.

Do any of you here have experience of both lenses and hence could proffer some advice on whether a swap would be a smart move. I don't normally photograph people across restaurant tables!

Re: Canon 50mm f1.4 v 85mm f1.2

Yes, I know, but the difference between 1.2 and 1.8 in terms of what I plan to use these lenses for may be significant. I am attracted to the usefulness of extremely shallow depth of field at times plus the ability to use the lens in very low light without pushing the ISO too far into the grain zone. I agree though that I can add the 85mm as well and still have £900 of change.

Re: Canon 50mm f1.4 v 85mm f1.2

Adrian: one important thing to think of in my way of thinking is the distance from the subject. If using an 85mm on the D5MK3, the distance is 20ft than the difference between the 1.2 and 1.8 is 6 inches in in DOF. The 1.2 at 10 feet your DOF would be approx. 4 inches, for me there is no way I would shoot a person with only 4 inches of focus depth and the closer I get the less depth I have to play with.
As you say the question was sharpness of 85 v 50 wide open both are excellent, the comprise is the shooter as you can not ask the camera to autofocus on the subject with that shallow DOF as it may not focus where you want it to, so you have to do it manually. So if you can focus that shallow then both are excellent and if you can not than again both are excellen,t it is you that are the comprise.

Re: Canon 50mm f1.4 v 85mm f1.2

Yes - I see your point Allen. In office based test shots of the 50mm 1.4 autofocus struggled at times. Manual focus was even more hit and miss unless I use live view and significant magnification. This is actually fine as most of the shots we do are from a tripod and we have enough time to get the focus right.

Actually, one of the reasons why I didn't order the 85 f1.2 at the outset was when I used the 24-105 zoom and set am 85mm point on this lens, we had a bit of trouble getting the camera far enough back in some situations.

Having tested the f1.4 50mm hand held quite a bit today, I do feel that at maximum aperture it is sharp-ish, as opposed to sharp. It's cheap so maybe I should keep it, as the lightness and smallness is handy and the absence of a red line does not shout "look at me" which can be a bad thing in some situations.

I might rent a 1.2L and just try before I buy. If I could be confident it is super sharp wide open I would risk it and buy one.

Re: Canon 50mm f1.4 v 85mm f1.2

I also have an 85mm f/1.2 (version I). Shooting wide open is for special effects not general photography. Here is an example of a test shot I took of my best friend right after I got the lens. I tried to focus on his closest eye. You can see how shallow the DOF is around the minimum focusing distance. It's a wonderful lens and very sharp although it is too slow to focus on a moving subject. The 1.8 version is much better for that (sports).

Re: Canon 50mm f1.4 v 85mm f1.2

Originally Posted by Adrian

I have been thinking about a prime for some shallow depth of field use, including portraiture. . . despite lusting after the 85mm f1.2 . . . I bought the 50mm f1.4 as it is pretty cheap (£220) and worth trying.
Now my dilemma. At f1.4 this lens will achieve very shallow depth of field (e.g. eyes in focus, ears pretty soft) but it is also quite soft overall. I have not carried out micro adjustment. Also autofocus is a bit erratic even in good light. Sharpness improves a lot by f2.8, but if I actually want very shallow depth of field, sharpness would be nice as well! . . . the question was sharpness of 85 v 50 wide open.

Do any of you here have experience of both lenses . . .

Yes. With the 50/1.4 and both versions of the 85/1.2
I’ll assume you are only considering the EF85 F1.2L MkII USM

My observations:

The 50/1.4 at F/1.4 is more “soft” than what the 85/1.2 when used at F/1.2.

Both lenses are softer at the edges than at the centre when used wide open – anecdotally, to my eye for real world shooting, both to the same degree of degradation form centre to edge

My 50/1.4 has no trouble auto focussing, in good to OK light, even in low light it Auto Focusses OK: it is necessary to understand how Canon's AF works - on contrast edges.

As a comment – the 50/1.4 is ‘quite sharp’ at the centre, when used wide open considering what it is and what it costs. The 85/1.2MkII is better in this regard and the price one pays for that degree of “better” is a lot of money and that expense is worth it for some. The EF85F/1.8 when used wide open falls between the two aforementioned lenses: and for my money represented value for money at the time I purchased – and that was before the 85/1.2MkII was released.

However I would now consider the EF85/1.2MkII as a lens for my purchase in the future, but as I already own the 85/1.8 I can spend money otherwise for the time being.

. . . the difference between 1.2 and 1.8 in terms of what I plan to use these lenses for may be significant. I am attracted to the usefulness of extremely shallow depth of field at times plus the ability to use the lens in very low light without pushing the ISO too far into the grain zone. I agree though that I can add the 85mm as well and still have £900 of change.

I suggest you might not know how significant or not significant the extra ⅓ Stop of Aperture Speed, will be to your quest

A few points:
Shallow DoF is not ‘achieved’ by the lens, sure I know what you mean that the Maximum Aperture will have some bearing, but that is the limit of the relationship.

The DIFFERENCE between the ‘shallowness’ of DoF when using an F/1.4 lens as opposed to an F/1.2 lens, for mostly ALL PORTRAITURE, is insignificant.

The DoF will remain constant, providing the framing is constant.

Here are the differentials, using the 85mm lens as an example, Portrait Orientation:

SO – even for a Full Length Shot, the DIFFERENCE in DoF if using F/1.4 or F/1.2 will be only 31mm (roughly 1¼inches) which for a visual representation is about the distance, front to back of the ear in the shot.

I understand that in the example above, I have compared F/1.4 to F/1.2 – and that is because your original question is about the 50/1.4 compared to the 85/1.2MkII.

If you are considering the 85/1.8, you can research the ⅔ Stop difference between F/1.2 and F/1.8 as iy applies to DoF – and the numbers will still reveal that the differential of DoF is NOT really that significant.
As one example we’ll cut straight to a Full length Shot, where the difference will be the greates of the series which I have presented:

The difference in DoF being 92.4mm is about 3⅝Inches . . . and relates to about the length of a middle finger . . . not that much difference, is it?

***

Originally Posted by Adrian

. . . plus the ability to use the lens in very low light without pushing the ISO too far into the grain zone.

⅔ Stop won’t make one iota of difference apropos the “the grain zone”.
⅓ Stop is even less significant in this regard.

***

Originally Posted by Adrian

and hence could proffer some advice on whether a swap would be a smart move

I’d suggest you base your evaluation for your purchase on aspects like overall image quality and price, rather than “shallow depth of field for Portraiture” or worrying about ISO.

Also if you are “evaluating” the 50/1.4: be quite sure what it is that you are evaluating.
Using that lens (any lens) hand held at apertures such as F/1.4 and on a subject who Is not secure - is often an evaluation in Shutter Timing; Hand-holding Techniques and the Stability of the Subject.
Such evaluations, across a table, or in any other real-life shooting scenario is usually NOT an evaluation the sharpness of the lens, the Auto Focus or an evaluation of the actual DoF.

Re: Canon 50mm f1.4 v 85mm f1.2

Originally Posted by Adrian

I have been thinking about a prime for some shallow depth of field use, including portraiture. I am aware that 50mm is not a classic length for portraits (this is on a full frame 5DIII by the way) but working indoors in confined spaces at times 50mm can be useful. I am quite happy to send the 50mm back IF the 85mm will be a big leap forward.

I reiterate that you should be aware what is being evaluated.

The 50 F/1.4 and some general comments on extreme LOW LIGHT Available Light Portrait Shooting which might assist you.

I have already detailed the (very small) Difference of DoF when using F/1.2 or F/1.4 or F/1.8 - and these examples below, might be valuable for you to apply so that you might better understand when you get shooting in low light with fast primes – what it actually is that you might be evaluating, when you look at the final shot.

Let's look at the lens first:

The 50/1.4 is a nice lens and is ‘relatively sharp’ at the centre, when used wide open.

And is quite sharp at F/2.2:

And the enlargements:

***

Let's now use the lens in low light to cover a real event

Using the 50/1.4 (or any other lens), close to or at wide open for low available light work, we need to understand that there are many more aspect to this particular genre of Photography, than worrying about the extra bit of Stop to alleviate “grain” or using the lens wide open to gain a more shallow DoF - we usually want all the DoF that we can get.

In the real world, low light shooting, with adequate technique and understanding of how AF works the 50 /1.4 will make good AF, consistently.

The limitations and resultant “errors” for this type of shooting will most usually be the photographer’s technique not being able to adequately overcome the difficulties of any particular shooting scenario – and that has nothing to do with the lens’s performance.

Here is the EF 50 F/1.4 used as a Portrait Lens on a 5D for real world, low light shooting and the commentary in each frame provides the tech specs and the condensed versions of the realistic evaluations of what actually happened for each shot.

WW

BTW - You are fortunate that you have the capacity to return the 50/1.4 after you have used it, simply because you have changed your mind.

Re: Canon 50mm f1.4 v 85mm f1.2

Dear Colin and William. Thank you for such helpful and comprehensive replies. This has been more useful than you may realise. I will reflect on it a bit and post a considered response later on. Best regards, Adrian

Re: Canon 50mm f1.4 v 85mm f1.2

OK, here is a more considered reply. What am I trying to achieve:

Leaving aside my personal photography, the need is to get reliable indoor shots of our staff, brochures and office related scenes that we can publish to our web site and sometimes print images in brochures. This is largely technical financial services material and the photography is in some respects incidental. However, imagery does need refreshing a lot in order to keep the ever changing website “alive”. In addition we need to record short daily video clips that we web publish.

Usually I am not the photographer. The equipment is a Canon 5DIII, two large LED lights (which we mainly bought for video as they are not too hot) is most often used by one of my staff, none of whom are trained in photography (nor is it their hobby), so I have to show them what is needed. Time is almost always a constraint. Therefore simplicity is important. Understanding autofocus options and methods is beyond their capability right now, and getting into metering options would be a step too far currently. So I need to set the camera up to be a good compromise.

We are mostly shooting in a difficult environment. Most of the work is indoors. In most areas the overhead modern fluorescent lighting is operated by motion detection and cannot be switched off. We can control the electric lights in some areas but these areas have space constraints and background distraction issues. A major issue is we have almost floor to ceiling windows around three sides of our office building. Most of our internal partitioning is also glass: either clear or frosted. This limits flexibility for plain wall backgrounds. Light is "all over the place" and very unpredictable.

To get nice, usable shots we are vey keen to blur out the background and for the bokeh to be pleasing (incidentally the f1.4 50mm lens did not thrill me in this respect when point light sources were in the background). Space issues mean that it is difficult physically to separate the subject from the background to any significant degree.

Practically all shooting is done from a tripod.

The reason why I mentioned low light performance and ISO was because we have really struggled with exposure for some shots. I find myself pushing the exposure and at times the ISO to get good shots or video. Part of this is not wanting to crowd the subject with the lights too closely: remember we are not using professional models here – the environment needs to be a natural and unobtrusive as possible.

I was very interested in your comments about the lack of use of the f1.2 in studio portraits and that has really made me think again. I was not actually comparing 1.2 against 1.4. The thought was 1.2 v 1.8. I may have inadvertently misled people here by rather shorthand descriptions. We don’t shoot much at very wide apertures as overall a pleasing sharp image is what we are after. Sometimes our web guys ask us for very shallow depth of field imagery – such as having only a few words in focus on a page of text on a desk. To do this without spending lots of time in PP it is easier if I can have very shallow DOF. However, this is without doubt only an occasional need.

We do want the portraits to be flattering and I am at a bit of a loss currently how to achieve this and have the background nicely out of focus when that background is still quite close (of necessity) to the subject.

For portraits we are shooting RAW plus JPEG and downloading into Aperture 3 on a calibrated (using i1 display pro) very high spec iMac. We can process through DxO elite and then Photo Professional 64 bit and we will shortly have Lightroom 4 and Photoshop available in cloud CS6. However, this is not our day job and there is a limit to how much time we can play around to get good imagery.

Most of our portraits and practically all of our video involves people sitting at a table singly, as a group of two or sometimes more. We also have some standing shots both full length and head and shoulders and sometimes informal “sofa” shots. Everything gets cropped and the IQ is totally fine for what we need.

Mostly we use a 24-105 f4 L. This is a good general purpose lens and it is plenty sharp enough. However, when we want to limit the DOF more, it is lacking. Bokeh is OK, not stellar. I have tried a longer zoom, but space constraints limit the usefulness unfortunately.

So I am now wondering what to do to overcome our challenges and deliver the best images with the least amount of work!

Doing this ourselves is a major cost saving for us. Licensing images is expensive (very expensive if you use Getty) and also we want to have a degree of exclusivity. Hiring a professional photographer has been disappointing on all three occasions we have tried it and it is not an option anyway for daily video.

It may be that what I am asking for is not really achievable! It could almost be summed up as I want to take perfect portraits confined spaces with erratic lighting, and I want blurred or invisible background but with the subject stood close to them...

Trial and error is inevitable. I am simply attempting to minimise the equipment error component.

Re: Canon 50mm f1.4 v 85mm f1.2

I think you really need to start with a good understanding of the fundamentals and then see what the best compromise is going to be -- and it IS going to be a compromise.

Some food for (random) thought ...

- If there's "light all around" then great - that's generally a plus. You'll invariable find that it's nowhere near as bright as it is outside though - so the first thing to think about is "is this the light we're going to use, or are we going to either supplement or replace it with our own lights?" In my experience, normal interior lighting is insufficient (by a significant margin) for quality photos of people without using additional lighting. You just can't use the apertures you need and still have a respectable shutter speed (even at higher ISOs)

- Following on from that - generally - LED lights like you describe aren't much better. Might be OK for video, but the term "farting against thunder" pretty much sums up their contribution to normal exposure requirements. So if at all possible, get some medium quality studio strobes and soft boxes in there.

- Keep in mind that what the camera sees ISN'T what the human eye sees. Case in point - in my studio I'm typically shooting at F11 @ 1/125th @ ISO 100. The studio measures 8 x 10 meters and has a total of 8 fluorescent 40 watt tubes. We normally leave the room lights on during shooting so we can see what we're doing and also as an assist for the auto-focus - but - if I take a shot at the above settings with out the studio strobes on then I get a TOTALLY back frame. The room lighting that lets us see the subject clearly doesn't so much as even register for the camera (just the way we like it!). The room lighting totals 320 watts - the instantaneous output from my studio strobes maxes out at around 6,000,000 watts. HUGE difference is an understatement. Basically, room lighting for quality portraiture sucks, unless you augment it.

- Often peoples reaction to the above is to "stick their heads in the sand" and jump for a fast lens - up the ISO - and take a looooow shutter speed - and end up with a combination of noise - insufficient DoF - and subject motion and/or camera shake. There is just no beating the laws of physics.

- With the natural lighting and LED lighting - and space you have - personally - I'd try a 24-70mm lens & shooting around F2.8 @ ISO 400 or 800 and let the cards fall where they may in terms of shutterspeed, but it's never going to give you the options that additional studio lighting would. You'll be pretty hit and miss.

- In terms of backgrounds, again, the laws of physics rule. First rule is to select an aperture that gets your subjects in focus - 2nd rule is to get the back ground as far away as possible - and for a given camera to subject distance that's about all you can do. Another option is to construct some kind of vague and "out of focus" background that you can just slide in behind them (props). Another technique is to just let the light levels fall away in comparison to what falls on the subjects (remember that regardless of how bright it may look to you, it's the camera settings that ultimately determine how it looks to the camera. If you can't control and use the natural lighting then just beat it into submission) (the term "I reject your reality and substitute my own comes to mind"!).

If it were me having to shoot in that environment - in the planning phase - I'd be investing about 2% of my time thinking about lenses (I'd use either my 24-70 2.8L or 70-200 2.8L IS II) - perhaps 28% on backdrops and props - and 70% (or more) on lighting. In the studio it's pretty much 80 to 90% time spent on lighting and the rest singing along to all time favourites playlist.

I'm not saying what you want can't be achieved by other means - but I do suspect that it's possibly "fear of the unknown" (ie lighting) that's pushing you more towards familiar "solutions" (lenses and settings), and from experience, that's ultimately a long and hard road with less progress.

Re: Canon 50mm f1.4 v 85mm f1.2

I think the other consideration is looking at the history of ultra-fast lenses. These originally tended to be aimed at press photographers who at times working in terrible lighting conditions and were shooting B&W film. Nicely said the ability to capture the image was far more important than a bit of softness in the overall image. Don't forget, ISO 400 was a fast film in those days and this could be pushed to ISO 800 and beyond at the cost of detail and grain. With an f/1 lens (thinking of the legendary Leitz Noctilux here; I had a chance to shoot one years ago and the thing that I still remember is that it weighed as much as the camera body), you could shoot at a full 2-stops faster than the standard f/2 lens. Stated another way, one could shoot ISO 100 film in a situation where ISO 400 film would have to be used with an f/2 lens.

With the ludicrously high ISO settings we get on our DSLRs, this advantage is long gone. Yes, shallow DoF can be achieved, but as Colin correctly points out, wide open it really is far too shallow for acceptable portraiture. Traditionally, lenses faster than around f/1.8 have been sharper and more aberration free than their faster counterparts; a wider maximum aperture means the light has to bend more, which means more lens elements are required, using more exotic glass, etc. which means a much higher cost for minimal technical advantages. The other advantage of the more expensive glass is that the hardware is more robust as well. I was looking at the f/1.8 and f/1.4 85mm Nikkors a couple of weeks ago, and the mechanical build quality of the f/1.4 was noticably better than the much less expensive and slightly slower lens. I strongly suspect the same can be said for the Canons.

The simplest and most cost effective solution is much the same as what I suggested when you were asking questions about shooting video. ADD MORE LIGHT! The relatively cost effective studio lights (and hot halogen lights for video) provide far better light quality and quantity for much less cost than some of the more exotic newer light sources. Rather than spending money on exotic glass, a less sexy solution that is going to give you far better results is to stick with the basics. Mount some seamless on the wall (Manfrotto has a really nice system) and roll it up and down as needed to eliminate the need to through the background out of focus. Set up a studio strobe with a softbox and reflector and you will get some really first class shots for your website with some reasonably priced glass.

Re: Canon 50mm f1.4 v 85mm f1.2

They are compact enough to fit in any of our meeting rooms or board rooms and they have the huge advantage for video that they are not hot, so our subjects don't get sweaty! However, we are not exactly in the 6 million watts league here.

We have used them for portrait shots and they are actually pretty good, especially when supplemented with some daylight.

A factor for us is set up time. At the moment we don't have a permanent studio (this may change soon) so my team has to set up and take down every time we want to shoot. It needs to be reasonably user friendly.

Do you think the LEDs are unsuitable? I realise that light drops off rapidly as the lights are moved away from the subject.

Reflectors are another issue. We want to take shots of people seated. We have several reflectors and use them to try to eliminate undesirable eye socket shadows etc. It is surprisingly difficult to have the reflector in an effective place as well as out of shot.

I will investigate studio strobes.

What put me off the 24-70 2.8L zoom was that it lacked IS (not relevant when on a tripod) and seemed to be doing pretty much the same job as the 24-105 but with a faster aperture wide open. Seemed like questionable value and since we avoid zooming for video anyway (very annoying to the watcher quite often!) I thought that primes might give a better result. Clearly I need to think more deeply about this.

On my personal list is acquisition of a decent macro lens and at the back of my mind is having a range of lenses between me and the business that can be borrowed from each other and will cover a range of jobs. This is such a steep learning curve! Mistakes waste everyone's time and slow down getting the results we want.

Re: Canon 50mm f1.4 v 85mm f1.2

Originally Posted by Adrian

Mistakes waste everyone's time and slow down getting the results we want.

The experience level on this board is very high and the advice is excellent. There are limits and it is no substitute for being there however.
May I suggest you get a local pro in to help you with the initial set-up. He/she/it can assess the environment, discuss with you the requirements, ask questions to clarify all with immediate responses.
Such a discourse (with a pro worthy of the name) will be far quicker than using this forum and fewer mistakes will occur.

Re: Canon 50mm f1.4 v 85mm f1.2

I didn’t realize that this thread is what amounts to a continuation of the previous and ongoing questions you have been asking. An example is here

On that previous thread, I was awaiting an answer to my questions. You have now provided some additional information.

***

In summary, to best to address ALL of your requirements and each requirement in the BEST manner possible:

I suggest you use the EF24 to 105F/4 IS for both the Stills Portraiture and also for the Video.

I suggest you use the LED lights for the Videos.

I suggest you buy a CANON dedicated Flash and use it ON CAMERA with a WHITE BOUNCE CARD for the Stills Portraits. Set the camera to: ISO 800; P Mode; Centre AF; Evaluative Metering; IS ON; and the Flash to ETTL II (Full AUTO) and ensure the one instruction you give the staff is to centre the camera on the chest of one of the subjects and shoot little wide for cropping later.

Re: Canon 50mm f1.4 v 85mm f1.2

If only we could Graham! We have tried three pros in total.

Number one struggled to produce images that were well composed, in focus and of high enough IQ. We had 400 high quality a and 400 low resolution images and hardly anything was usable. She was however very personable.

Pro number 2 came highly recommended but within a short time her patronising manner had upset several staff. The consensus from my team was that she produced unflattering portraits that made people look ten years older. Given that the average age of our staff is below 30, this was quite an achievement.

Pro number 3 had signed a contract which clearly stated that copyright remained with us and that he could not sell or otherwise use any images. When we got the images they all had his copyright logo plastered across them, which he was very reluctant to remove. He also arrived two hours late for the start of our shooting day because he overslept. This guy did not get paid.

Our general view was that the "pros" were lazy and somewhat under equipped. None of them were by any means cheap.
The truth is that finding good "corporate" photographers is much harder than you might think. All three of them sneered at video, which seemed almost childish in one case. Pro photographers need to embrace the digital media age.

We are in central London - hardly remote. It was this set of experiences that led us to go it alone. We actually enjoy this and the team likes the challenge of being creative. We have been pleased with many of our images and found it very satisfying to embark on this journey of learning. It is also very flexible: if we need an image we produce it there and then.

We would rather buy our own equipment than waste another £1500 or £2000 on a "pro". So far I have been able to get all of the images I want, both portraits, office shots and external building and model shots. But I am pushed for time as I actually have a business to run! Hence I am trying to spread the load and ease the workflow.

Re: Canon 50mm f1.4 v 85mm f1.2

Originally Posted by Adrian

Do you think the LEDs are unsuitable? I realise that light drops off rapidly as the lights are moved away from the subject.

Well ...

... to give you an idea ... many years ago I thought I could do lighting "on the cheap" so I bought some halogen work lights. I'm a bit confused as to how powerful they were - I think there were 4 x 500W (2000W in total), but I also remember having to power them from different circuits so I didn't trip the circuit breaker - so maybe more.

I think I used ISO 400 and F8 - and I was STILL down to only 1/20th. In a word, they were hopeless as a light source (they sure heated the room up quickly though) - and I'm sure they would have been brighter than your LEDs. For a good quantity of light you just can't beat a strobe.

What put me off the 24-70 2.8L zoom was that it lacked IS (not relevant when on a tripod) and seemed to be doing pretty much the same job as the 24-105 but with a faster aperture wide open. Seemed like questionable value and since we avoid zooming for video anyway (very annoying to the watcher quite often!) I thought that primes might give a better result. Clearly I need to think more deeply about this.

The 24 to 105 is also a great lens - horses for courses though. If you go with the strobes then the 24-105 will be fine - if you stick with LED lights then you'll probably need that extra stop at F2.8. Interestingly, Canon have at long last released a new version of the 24-70 - and it STILL doesn't have IS. Many were surprised by this, but in reality, its just not as beneficial at shorter focal lengths. It's more of a "professional's choice" lens - and just about every professional would also have the EF70-200 F2.8L IS USM II - so in a professional sense one can think of the 24-70 and 70-200 as being "parts B & C of the trilogy of lenses" (part A is the EF16-35 F2.8L USM).

Personally (although others are sure to disagree!), I can't see any advantages in you using a prime; there will be no real-world difference in sharpness - any difference in max aperture you won't be able to use due to DoF considerations - and of course you'll be limited to only 1 focal length.